CATADIOPTRIC PROJECTION SYSTEM FOR 157nm LITHOGRAPHY

ABSTRACT

A photolithography reduction projection catadioptric objective includes a first optical group including an even number of at least four mirrors, and a second at least substantially dioptric optical group more imageward than the first optical group including a number of lenses for providing image reduction. The first optical group provides compensative axial colour correction for the second optical group.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/771,986, filed Feb. 3, 2004, which is a continuation of U.S. patentapplication Ser. No. 10/444,897, filed May 23, 2003, which is (1) acontinuation of International Application Serial No. PCT/EP01/13851,filed Nov. 28, 2001 and published in English on Jun. 6, 2002, whichclaims priority from U.S. Provisional Patent Application Ser. No.60/253,508, filed Nov. 28, 2000 and from U.S. Provisional PatentApplication Ser. No. 60/250,996, filed Dec. 4, 2000, and (2) aContinuation-in-Part of U.S. patent application Ser. No. 09/761,562,filed Jan. 16, 2001 (now U.S. Pat. No. 6,636,350) which claims thebenefit of priority to U.S. Provisional Patent Application No.60/176,190, filed Jan. 14, 2000, all of the aforementioned patentapplications and patents are hereby incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to projection systems for photolithography, andparticularly to catadioptric systems including first and second opticalimaging groups for 157 nm lithography.

2. Discussion of the Related Art

Extending DUV lithography to sub 100-nm linewidths requires a projectionsystem with a high numerical aperture, e.g., 0.65-0.75 or larger, at awavelength of 157 nm. As optical lithography is extended into the vacuumultraviolet (VUV), issues surrounding the laser linewidth and materialavailability could cause substantive delays to the development of acommercial 157 nm step/repeat or step/scan tool. Therefore, it isdesired to investigate optical configurations that minimize theconsumption of calcium fluoride.

Microlithographic reduction projection catadioptric objectives, such asthat shown and described with respect to FIG. 3 of European patentapplication no. EP 0 779 528 A2, which is hereby incorporated byreference, may be understood as variants of pure catoptric objectives.FIG. 3 of the '528 application shows a system having six minors andthree lenses. The optical surfaces are generally symmetric to a commonaxis, and the object plane and the image plane are situated on this sameaxis upstream and downstream of the objective, respectively. Asdescribed in the '528 application, the system of FIG. 2 therein has anumerical aperture of only 0.55 and that of FIG. 3 therein only 0.6. Inaddition, all but one of the six mirrors shown at FIG. 3 are cut offsections of a bodies of revolution, yielding mounting and adjustmentface difficulties. Also, the lenses shown in FIG. 3 serve only ascorrecting elements having minor effect. In addition, the most imageward(or optically closest to the image plane) mirror described in the '528application is concave. It is desired to have an objective with a highernumerical aperture, and which is constructed for easier mounting andadjustment.

A similar objective to that described in the '528 application (above) isdisclosed at U.S. Pat. No. 4,701,035, which is hereby incorporated byreference. The objective shown at FIG. 12 of the '035 patent, forexample, has nine mirrors, two lenses and two intermediate images. Theobject plane and image plane are situated within the envelope of theobjective. The objective described in the '035 application also exhibitsa low numerical aperture and offers similar mounting and adjustmentdifficulties as described above with respect to the '528 application. Inboth the '528 and '035 applications, the image field is an off-axis ringsector.

An axially symmetric type of catadioptric objective is disclosed inGerman patent document DE 196 39 586 A (see also U.S. patent applicationSer. No. 09/263,788), each application of which is hereby incorporatedby reference. The '586 application discloses an objective having twoopposing concave mirrors, an image field centered at the common axis anda central obscuration of the aperture. It is recognized herein that itis desired to have an axially symmetric objective having an unobscuredaperture. Another type of catadioptric objective for microlithographicreduction projection has only one concave mirror and a folding mirror,as is described at U.S. Pat. No. 5,052,763 and European patentapplication no. EP 0 869 383 A, which are each hereby incorporated byreference.

It is recognized herein that catadioptric optical systems have severaladvantages, especially in a step and scan configuration, and that it isdesired to develop such systems for wavelengths below 365 nm. Onecatadioptric system concept relates to a Dyson-type arrangement used inconjunction with a beam splitter to provide ray clearance and unfold thesystem to provide for parallel scanning (see, e.g., U.S. Pat. Nos.5,537,260, 5,742,436 and 5,805,357, which are incorporated byreference). However, these systems have a serious drawback since thesize of this beam splitting element becomes quite large as the numericalaperture is increased up to and beyond 0.65 to 0.70, making theprocurement of bulk optical material with sufficient quality (inthree-dimensions) a high risk endeavor. This problem is exacerbated aswavelengths are driven below 193 nm because the selection of materialthat can be manufactured to lithographic quality is severely limited.

To circumvent this problem, it is recognized herein that it is desiredto develop systems without beamsplitters. However, it is difficult toachieve an adequately high numerical aperture (e.g., U.S. Pat. Nos.4,685,777, 5,323,263, 5,515,207 and 5,815,310, which are incorporated byreference), or to achieve a fully coaxial configuration, instead ofrelying on the use of folding mirrors to achieve parallel scanning(e.g., U.S. Pat. No. 5,835,275 and EP 0 816 892, which are incorporatedby reference) and thereby complicating the alignment and structuraldynamics of the system. In addition, it is desired to have an opticaldesign that generally does not utilize too many lens elements, which cangreatly increase the mass of the optical system.

WO 01/51 979 A (U.S. Ser. No. 60/176,190 and 09/761,562) and WO 01/55767A (U.S. Ser. No. 60/176,190 and 09/759,806)—all commonly owned andpublished after the priority date of this application—show similarcoaxial catadioptric objectives with 4 mirrors or more.

EP 1 069 448 A1 published after the priority date of this applicationshows a coaxial catadioptric objective with two curved mirrors and areal. intermediate image located besides the primary mirror.

All cited publications are incorporated herein by reference in theirentirety. It is desired to develop a compact, coaxial, catadioptricprojection system for deep ultraviolet and/or vacuum ultravioletlithography that uses no beamsplitters or fold mirrors in is opticalpath.

It is an object of the invention to provide an objective formicrolithographic projection reduction having high chromatic correctionof typical bandwidths of excimer laser light sources, which permits ahigh image-side numerical aperture, and which reduces complexity withrespect to mounting and adjusting.

SUMMARY OF THE INVENTION

In view of the above, a photolithography reduction projectioncatadioptric objective is provided including a first optical groupincluding an even number of at least four mirrors, and a second at leastsubstantially dioptric optical group more imageward than the firstoptical group including a number of lenses for providing imagereduction. The first optical group provides compensative axial colourcorrection for the second optical group.

A preferred embodiment is a photolithographic reduction projectioncatadioptric objective including a first optical group including an evennumber of at least six mirrors, and a second at least substantiallydioptric optical group more imageward than the first optical groupincluding a number of lenses for providing image reduction. Thisincreased number of mirrors gives more degrees of freedom to thecorrection and simplifies the design for stressed qualities.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the lens section of a projection objective for 157 nmphotolithography according to a first preferred embodiment.

FIG. 2 shows the lens section of a second preferred embodiment.

INCORPORATION BY REFERENCE

What follows is a cite list of references which are, in addition to thereferences cited above in the background section, hereby incorporated byreference into the detailed description of the preferred embodiment, asdisclosing alternative embodiments of elements or features of thepreferred embodiment not otherwise set forth in detail herein withreference to FIG. 1 or FIG. 2. A single one or a combination of two ormore of these references may be consulted to obtain a variation of thepreferred embodiment described above. Further patent, patent applicationand non-patent references, and discussion thereof, cited in thebackground and/or elsewhere herein are also incorporated by referenceinto the detailed description of the preferred embodiment with the sameeffect as just described with respect to the following references: U.S.Pat. Nos. 5,323,263, 5,515,207, 5,052,763, 5,537,260, 4,685,777,5,071,240, 5,815,310, 5,401,934, 4,595,295, 4,232,969, 5,742,436,5,805,357, 5,835,275, 4,171,871, 5,241,423, 5,089,913, 5,159,172,5,608,526, 5,212,588, 5,686,728, 5,220,590, 5,153,898, 5,353,322,5,315,629, 5,063,586, 5,410,434, 5,956,192, 5,071,240, 5,078,502,6,014,252, 5,805,365, 6,033,079, 4,701,035 and 6,142,641; and Germanpatent no. DE 196 39 586 A; and U.S. patent applications Ser. No.09/263,788 and 09/761,562; and European patent applications No. EP 0 816892 A1, EP 0 779 528 A2 and EP 0 869 383 A; and

“Design of Reflective Relay for Soft X-Ray Lithography”, J. M. Rodgers,T. E. Jewell, International Lens Design Conference, 1990;

“Optical System Design Issues in Development of Projection Camera forEUV Lithography”, T. E. Jewell, SPIE Volume 2437, pages 340-347;

“Ring-Field EUVL Camera with Large Etendu”, W. C. Sweatt, OSA TOPS onExtreme Ultraviolet Lithography, 1996;

“Phase Shifting Diffraction Interferometry for Measuring ExtremeUltraviolet Optics”, G. E. Sornargren, OSA TOPS on Extreme UltravioletLithography, 1996; and

“EUV Optical Design for a 100 nm CD Imaging System”, D. W. Sweeney, RHudyma, H. N. Chapman, and D. Shafer, SPIE Volume 3331, pages 2-10

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A catadioptric projection system according to a preferred embodimentherein is schematically shown at FIG. 1 and includes two distinctoptical groups G1 and G2. Group G1 is a catadioptric group includingmirrors M1-M6 and lenses E1-E3, as shown in FIG. 1. An object or maskplane Ob is disposed to the left of group G1 in FIG. 1 or opticallybefore group G1. Group G2 is disposed optically after group G1 and tothe right of group G1 in FIG. 1. An image or wafer plane Im is disposedoptically after group G2 and to the right of group G2 in FIG. 1.

Group G1 functions by correcting field aberrations and providing aconjugate stop position for correction of axial chromatic aberration.Group G2 is a dioptric group including lens elements E4-E13, as alsoshown in FIG. 1. Group G2 lies aft of G1, or optically nearer the imageplane of the system, enabling the system to achieve numerical aperturesin excess of 0.65, 0.70 and even 0.75. This catadioptric system achievesa high numerical aperture preferably using no beamsplitters nor foldmirrors. The description herein examines the performance of thepreferred system of FIG. 1.

As mentioned, the system of FIG. 1 is separated into two optical groups,i.e., group G1 including 6 mirrors and 3 lens elements and group G2including 10 individual lens elements. The design is purely coaxial witha single common centerline (axis of symmetry) using an off-axis field toachieve the necessary ray clearance so that the mask and wafer planesare parallel, Group G1 forms a virtual image V1 located behind mirror M6at a reduction of ˜0.8x. Group G2 takes this virtual image and forms ausable real image at the wafer. Group G2 takes this virtual image andforms a usable real image at the wafer. Group G2 operates at a reductionof about 0.25×, allowing the system to achieve a desired reduction of0.20×. A complete optical prescription is found in Table 2, below,describing the optical surfaces in Code V format.

Referring to FIG. 1, how the preferred design achieves the performancelisted in Table 1 is now explained. To correct chromatic aberration, theaperture stop AS that lies in group G2 has a conjugate position locatedwithin group G1 preferably at, and alternatively near, mirror M2. At M2,strong negative lenses E2/B3 are used in a double-pass configuration forinducing overcorrected (positive) axial chromatic aberration used tobalance or correct an undercorrected (negative) axial chromaticaberration created by the strong positive optical power of group G2.With regard to lateral chromatic aberration, FIG. 1 shows an aperturestop AS in group G2 placed in a quasi-symmetric manner, allowing theLateral chromatic aberration to be at least nearly self-corrected withingroup G2 itself. In practice, lateral chromatic aberration of a fewparts per million (ppm) may be within tolerance within group G2 and canbe corrected using slight asymmetry of the chief ray near the conjugatestop position at mirror M2.

By balancing aberration correction between G1 and G2, the monochromaticaberrations are corrected in such a way to leave the lens elementswithin G2 “unstressed.” The term “unstressed” is used to signify thefact that no steep ray bendings are used within G2 to promote high-orderaberration correction. Both the chief and marginal rays exhibit thisbehavior. The fact that group G2 is “unstressed” is advantageous whenmanufacturing and assembly tolerances are considered in detail.

Overall, the system of FIG. 1 includes 6 mirrors and 13 lens elements ina coaxial configuration all coaxial to axis A. The design utilizes anoff-axis field to enable ray clearance and allow the mask and waferplanes to be parallel. Lens element E1 of group G1 is used to make thechief ray telecentric at the mask plane. Group G1 forms a virtual imagebehind mirror M6, which is relayed by the dioptric group G2 to form afinal image at the wafer plane. TABLE 1 System of FIG. 1 PerformanceSummary Parameter Performance Wavelength (nm) 157 Spectral band (pm) 0.5Reduction ratio (R) 0.20 Field size (mm) 22 × 7 rectangular Numericalaperture (NA) 0.75 RMS wavefront error (waves) 0.013 λ Distortion (nm)<1 nm PAC (ppm) 39.0 ppm PLC (ppm)  0.0 ppm Total track (mm) distanceOb-Im 1250 Front working distance (mm) 25.0 Back working distance (mm)10.0 Blank mass (kg, estimated) 39.0

Table 1 shows that the monochromatic RMS wavefront error, distortion,and chromatic aberrations PAC—paraxial axial colour aberration andPLC—paraxial local colour aberration are reduced small residual valuesas desired for precision lithographic projection systems. Further, thesystem of FIG. 1 may be confined within a volume that is similar to orsmaller than conventional systems, meaning that the footprint of legacytools can be maintained, if desired. TABLE 2 Optical Design Prescriptionfor the System of FIG. 1 RDY THI RMD GLA OBJ: INFINITY 25.000000  1:INFINITY 0.000000  2: INFINITY 0.000000  3: 329.41693 30.000000‘cafl_vuv’ ASP: K: 0.722126 A: 0.000000E+00 B: −.225942E−11 C:0.167998E−15 D: −.128550E−20 E: −.233823E−24 F: 0.685735E−29 G:0.000000E+00 H: 0.000000E+00  4: 502.56913 59.208438  5: INFINITY347.586957  6: −1183.47149 −347.586957 REFL ASP: K: A: −.127089E−08 B:0.812330E−14 C: −.123118E−18 D: 0.894383E−23 E: −.276494E−27 F:0.402755E−32 G: 0.000000E+00 H: 0.000000E+00  7: 279.62176 −7.500000‘cafl_‘vuv’  8. 745.02111 −5.835889  9. 350.74458 −7.500000 ‘cafl_‘vuv’10. 1226.35940 −8.372549 11. 324.93068 8.372549 REFL ASP: K: 0.069031 A:−551054E−09 B: −.166403E−13 C: −.307699E−18 D: 0.277748E−22 E:−.680019E−26 F: 0.506026E−30 G: 0.000000E+00 H: 0.000000E+00 12:1226.35940 7.500000 cafl_‘vuv’ 13: 350.74458 5.835889 14: 745.021117.500000 ‘cafl_‘vuv’ 15: 279.62176 304.397688 16: 490.28038 −244.852865REFL ASP: K: −1.803201 A: −.482804E−08 B: −.125400E−12 C: 0.242638E−17D: −.680221E−22 E: 0.237919E−26 F: −.315262E−31 G: 0.000000E+00 H:0.000000E+00 17: 667.70113 565.726496 REFL ASP: K: −0.118347 A:−.275181E−09 B: −.327224E−14 C: 0.200875E−19 D: −.620470E−24 E:0.627048E−29 F: −.394543E−34 G: 0.000000E+00 H: 0.000000E+00 18:INFINITY 25.997938 SLB: “Intermediate image” 19: −1126.18103 −178.682300REFL ASP: K: 7.738777 A: −.668802E−08 B: 0.253685E−12 C: −.548789E−17 D:0.625386E−22 E: −.276305E−27 F: −120188E−33 G: 0.000000E+00 H:−0.000000E+00 20: −1002.36339 178.682300 REFL ASP: K: 50.616566 A:−973184E−08 B: 0.308396E−12 C: −.511443E−16 D: 0.428520E−20 E:−217208E−24 F: 0.518418E−29 G: 0.000000E+00 H: 0.000000E+00 21: INFINITY−324.644282 22: INFINITY 324.644282 SLB: “Virtual image” 23: INFINITY139.926509 24: 532.50558 30.000000 ‘cafl_vuv ASP: K: −28.969955 A:0.000000E+00 B: −.109172E−I1 C: 0.625819E−16 D: −.274325E−20 E:0.634878E−25 F: 0.581549E−29 G: 0.000000E+00 H: 0.000000E+00 25:−584.92060 2.500000 26: 1292.88867 13.668481 ‘cafl_‘vuv’ 27: −1383.773412.500000 28: 760.97648 15.674455 ‘cafl_‘vuv’ 29: −1077.75076 11.00142130: −250.22566 10.000000 ‘cafl_vuv’ 31: −500.99843 11.138638 STO:INFINITY 22.619203 SLB: “stop” 33: −298.09900 18.822972 ‘cafl_vuv’ ASP:K: 6.689541 A: 0.000O00E+00 B: 0.346206E−12 C: −498302E−17 D:0.272385E−20 E −.106617E−24 F: 0.175645E−28 G: 0.000000E+00 H:0.000000E+00 34: −1073.42340 0.500000 35: 267.47103 50.000000 ‘cafl_’vuv’ 36: −607.58973 0.592125 37: 258.S1S26 27.182889 ‘cafl_’vuv’ 38:−8945.70709 0.500000 39: 159.70628 39.768717 ‘cafl_’vuv’ ASP: K:−1.214880 A: 0.000000E+00 B: −.252828E−11 C: −.632030E−16 D:−.765024E−21 E: 0.477017E−24 F: −.163970E−28 G: 0.000000E+00 H:0.000000E+00 40: −746.03878 0.500000 41: 122.36092 43.154424‘‘cafl_’vuv’ 42: 95.77143 4.340799 ASP: K: 1.012065 A: 000000E+00 B:0.214891E−12 C: −.187071E−14 D: −.681922E−18 E: 0.313376E−22 F:O.000O00E+00 G: 0.000000E+00 H: 0.000000E+00 43: 115.81595 30.082531‘‘cafl_’vuv’ 44: −1828.47137 9.930603 IMG: INFINITY 0.000000The catadioptric projection system according to a second preferredembodiment herein is schematically shown at FIG. 2 and includes twodistinct optical groups G1′ and G2′. Group G1′ is a catadioptric groupincluding mirrors M1′-M6′ and lenses E1′-E3′, as shown in FIG. 2. Anobject or mask plane Ob′ is disposed to the left of group G1′ in FIG. 2or optically before Group G1′. Group G2′ is disposed optically aftergroup G1′ and to the right of G′ in FIG. 2. An image or wafer plane Im′is disposed optically after group G2′ and to the right of group G2′ inFIG. 2.

Group G1′ functions by correcting field aberrations and providing aconjugate stop CS′ position for correction of axial chromaticaberration. Group G2′ is a dioptric group including lens elementsE4′-E13′, as also shown in FIG. 2. Group G2′ lies aft of G1′, oroptically nearer the image plane Im′ of the system, enabling the systemto achieve numerical apertures in excess of 0.65, 0.70 and even 0.75.This catadioptric system achieves a high numerical aperture preferablyusing no beamsplitters nor fold mirrors. The description herein examinesthe performance of the second preferred embodiment of FIG. 2.

The first embodiment of FIG. 1 features independent correction oflateral chromatic aberration in the individual imaging groups. Thisfeature influenced the optical construction in terms of stopposition(s), element powers and element shapes. In the present secondembodiment, the independent lateral color correction feature is notincluded and a balance of lateral color is struck between the fore andaft groups.

Group G1′ is a catadioptric group that functions by correcting fieldaberrations and providing a conjugate stop position to correct axialchromatic aberration. Group G2′ is a dioptric group that lies aft of G1′enabling the system to achieve numerical apertures(NA) in excess of0.65, and preferably at least 0.70, or 0.75, or even 0.80 or higher. Forexample, a system in accord with the preferred embodiment may beconfigured to exhibit a NA of 0.79 while advantageously having a RMSwavefront error of only 0.0115 λ. That is, the system may be configuredwith a NA above 0.75, while maintaining the RMS wavefront error below0.02 λ, and even below 0.015 λ.

The system shown in FIG. 2 has two distinct groups, as mentioned above.Group G1′ includes an even number of at least four mirrors, andpreferably has six mirrors M1′-M6′. Group G1′ further preferablyincludes three lens elements E1′-E3′. Group G2′ includes a lens barrelof ten individual lens elements E4′-E13′, as shown in FIG. 2. The designis coaxial having a single common centerline, respectively, of thesystem of two optical groups G1′ and G2′ shown in FIG. 2. The designuses an off-axis field to achieve ray clearances in group G1′ SinceGroup G2′ is dioptric, ray clearance problems are eliminated enabling asystem with a high numerical aperture. The concept also provides forunlimited scanning of the mask and wafer in a parallel configuration.

Group G1′ of FIG. 2 forms a minified, virtual image V1′ located behindmirror M6′ at a reduction of ˜0.8×. Group G2′ relays this virtual imageV1′ to form a usable real image Im at the wafer. Group G2′ operates at areduction of about 0.25x. allowing the system to achieve a reduction of0.20×. A complete optical prescription is found in Table 5 below,describing the optical surfaces in Code V format.

To correct chromatic aberration, the aperture stop AS′ that lies ingroup G2′ has a conjugate stop CS′ position in group G1′ between mirrorM1′ and M2′. This allows a negative chief ray height at elements E2′ andE3′ (for positive field height at the reticle (Ob′)). This chief rayheight, when combined with the sign of the marginal ray and the negativepower of the E2′/E3′ pair, provides for a lateral chromatic aberrationcontribution that substantially cancels the lateral color contributionfrom group G2′. Assuming a spectral bandwidth of 0.5 pm, this specificembodiment has a paraxial lateral color contribution from E2′/E3′ of ˜35ppm, whereas the paraxial lateral color contribution from Group G2′ is˜35 ppm, resulting in an advantageous sum total of approximately 0 ppm.The principle result is that the power distribution and shapes of thelenses in group G2′ take on a very advantageous form.

FIG. 2 also specifically shows raytrace layout of the preferredembodiment. The system shown includes six mirrors M1′-M6′ and thirteenlens elements E1′-E3′ in a coaxial configuration. The design utilizes anoff-axis field (ring field, rectangular slit field or the like) toenable ray clearance and allow the mask and wafer planes Ob′, Im′ to beparallel. Element E1 is preferably used advantageously to make the chiefray telecentric at the mask plane Ob′, as described in more detailbelow. Group G1′ forms a virtual image V1′ behind mirror M6′, which isrelayed by dioptric group G2′ to form the final image at the wafer planeIm′. A real intermediate image Im′ is also formed between mirrors M4′and M5′ of group G1′, as shown in FIG. 2.

At mirror M2′, negative lenses E2′/E3′ are used in a double-passconfiguration to induce overcorrected (positive) axial chromaticaberration used to correct undercorrected (negative) axial chromaticaberration created by the strong positive optical power of group G2′.The monochromatic aberrations are corrected via a balance between groupsG1′ and G2′. In addition, this is done in such a manner as to leave thelens elements E4′-E13′ in group G2′ “unstressed” as in the firstembodiment.

Lens element E1′ provides for the telecentric condition at the plane Ob′of the mask. It is advantageous to have positive optical power near themask to reduce the chief ray height on mirror M1′. Lens element E1′appears to lie in conflict with the substrate of mirror M2′. To achievethis concept, it is preferred that only a small off-axis section of E1′be used. This means that pieces of a complete E1′ could be sectioned toyield pieces for multiple projection systems, further reducing therequired blank mass of a single system.

Another option to resolve the apparent conflict between lens E1′ and thesubstrate of mirror M2′ is to place lens E1′ between mirrors M1′ andM2′, such as somewhere close to the group of lens elements E2′/E3′. Inthis Manner, the complete lens would be used. TABLE 3 PerformanceSummary of System of FIG. 2 Parameter Performance Wavelength (nm) 157Spectral band (pm) 0.5 Reduction Ratio (R) 0.20 Field size (mm) 22 × 7Numerical aperture(NA) 0.75 RMS wavefront error(waves) 0.006λ Distortion(nm) <2 nm PAC (ppm) 42.0 ppm PLC (ppm)  0.7 ppm Total track (mm) 1064Front working distance(mm) 28.0 Back working distance(mm) 8.7 Blank mass(kg, estimated) 34.4

Table 3 summarizes design performance of the system of the preferredembodiment. The system has a composite RMS wavefront error of 0.006 λ,with NA=0.75, evaluated monochromatically over the field. The distortionis less than 2 nm at all field points, and the lateral color PLC iscorrected to better than 1 nm. The axial color PAC is also small andcould be reduced further if desired and as understood by those spilledin the art. This design approaches an advantageous “zero aberration”condition. TABLE 4 Composite RMS wavefront error vs. NA NA RMS wavefronterror 0.75 0.0058λ 0.76 0.0061λ 0.77 0.0064λ 0.78 0.0075λ 0.79 0.0115λ0.80 0.0207λ 0.81 0.0383λ 0.82 0.0680λ

As desired dimensional specifications of IC manufactures shrink, thenumerical aperture may be advantageously scaled in accord with thepreferred embodiment. Table 3 illustrates how the design of FIG. 1 maybe scaled as the numerical aperture is increased. A local minimum thatdoes not scale well with aperture is preferably avoided, since otherwiseto achieve increased numerical aperture would involve additionalredesign. The aperture scaling of the preferred embodiment illustratedat FIG. 1 is presented in Table 3, above. From a qualitative standpoint,the table reveals that the preferred embodiment herein scales well withnumerical aperture. For example, the composite RMS only grows by 0.005 λfrom 0.0058 λ to 0.0115 λ as the NA is scaled from 0.75 to 0.79. Theresults indicate that the system of the preferred embodiment my bescaled to a numerical aperture larger than 0.80. TABLE 5 Optical DesignPrescription of System of FIG. 2 RDY THI RMD GLA OBJ: INFINITY 28.000000 1: INFINITY 0.000000  2: INFINITY 0.000000  3: 256.21415 19.957583‘cafl_vuv’  4: 461.83199 42.954933  5: INFINITY 329.408468  6:−947.39721 −329.408468 REFL ASP: K: 10.217685 A: −.271423E−08 B:0.413774E−13 C: 0.119957E−17 D: 0.566939E−22 E: −.201485E−26  7:235.67059 −5.250000 ‘cafl_vuv’  8: 1202.79595 −18.801014  9: 199.92931−5.250000 ‘cafl vuv’ 10: 471.74620 −10.153919 11: 245.63551 10.153919REFL ASP: K: 0.060091 A: 0.624853E−09 B: 0.113020E−13 C: −.515404E−18 D:O.I70604E−21 E = .159226E−25 F: 0.105279E−29 12: 471.74620 5.250000‘cafl_vuv’ 13: 199.92931 18.801014 14: 1202.79595 5250000 ‘cafl_vuv’ 15:235.67059 298.515259 16: 490.36196 −227.868676 REFL ASP: K: 0.133019 A:−.401120E-OS B: −.925737E−13 C: −.236166E−17 D: 0.108790E−21 E:−.551175E−26 F: 0.127289E−30 17: 6I1.66355 331.489215 REFL ASP: K:−0.837736 A: 0.918739E−11 B: −.476080E−14 C: 0.346155E−19 D:−.225369E−23 E: 0.307373E−28 F: −.248704E−33 18: INFINITY 126.863525 19:−561.20466 −126.090855 REFL ASP: K: 2.976905 A: −.I54058E−09 B:0.125754E−13 C: 0.647835E−19 D: 0.684380E−23 E: −.112193E−27 F:0.122096E−32 20: 278.57130 126.090855 REFL ASP: K: 8.694109 A:−.272648E−07 B: 0.129115E−12 C: −.101751E−15 D: 0.402887E−19 E:−.610026E−23 F: 0.531569E−27 21: INFINITY −226.338582 22: INFINITY226.338582 23: INFINITY 52.284606 24: 729.88242 21.000000 ‘cafl_vuv’ASP: K: −31.964685 A: O.000O00E+00 B: 0.562441E−11 C: 0.152848E−16 D:−.915976E−20 E: 0.259148E−24 F: 0.238241E−28 25: 158.15364 17.296741 26:1355.83270 24.560562 ‘cafl_vuv’ 27: −210.48464 0.700000 28: 376.4514923.959662 ‘cafl_vuv’ 29: −356.27423 15.713419 30: −132.60708 38.500000‘cafl_vuv’ 31: −152.06343 0.700000 STO: INFINITI′ 12.628192 33:273.21370 38.500000 ‘cafl_vuv’ ASP: K: 5.588882 A: O: 000O00+00E B:0.113851E−11 C: 0.272852E−16 D: 0.288236E−20 E: 0.101289E−24 F:0.171576E−25 34: −276.08617 0.700000 35: 240.81764 38.500000 ‘cafl_vuv’36: −48844.10806 11.186318 37: 164.33601 38.500000 ‘cafl_vuv’ 38:−2168.86405 2.528995 39: 157.43497 38.500000 ‘cafl vuv’ ASP: K:−1.250301 A: O.000O00E+00 B: −.532791E−11 C: −.258778E−15 D:−.139880E−19 E: 0.252524E−23 F:: 138502E−28 40: 29770.37524 2.727081 41:130.31599 33.479292 ‘cafl_vuv’ 42: 54.66735 3.097821 ASP: K: 0.179565 A:O.000O00E+00 B: 0.129145E−I 1 C: −.283430E−14 D: .650118E−17 E:0.238362E−20 43: 108.48722 20.284450 ‘cafl_vuv’ 44: INFINITY 8.741020IMG INFINI11′ 0.000000 q SPECIFICATION DATA NAO 0.15000 TEL DIM MM WL157.63 157.63 157.63 REF 21 1 WTW 1 1 1 XOB 0.00000 0.00000 0.000000.00000 0.00000 YOB 66.50000 75.25000 84.00000 92.75000 101.50000 YOB1.00000 1.00000 1.00000 1.00000 . 1.00000 VUY 0.00000 0.00000 0.000000.00000 0.00000 VLY 0.00000 0.00000 0.00000 0.00000 0.00000 REFRACTNEINDICES GLASS CODE 157.63 ‘cafl_vuv’ 1.559288 No solves defined insystem INFINITE CONJUGATES EFL −21643.8522 BFL −4320.0292 FFL 0.1082E+06FNO 0.0000 AT USED CONJUGATES RED −0.2000 FNO −0.6667 OBJ DIS 28.0000 TT1064.0000 IMG DIS 8.7410 OAL 1027.2590 PARAXIAL IMAGE HT 20.3000 THI8.7412 ANG 0.0000 ENTRANCE PUPIL DIA 0.3034E+10 THI 0.1000E+11 EXITPUPIL DIA 6567.5310 TFiI −4320.0760 SPECIFICATION DATA NAO 0.15000 TELDIM MM WL 157.63 REF 1 WTW 1 XOB 0.00000 0.00000 0.00000 0.00000 0.000000.00000 0.00000 0.00000 0.00000 YOB 100.00000 107.50000 115.00000125.50000 130.00000 105.00000 110.00000 120.00000 125.00000 YOB 1.000001.00000 1.00000 1.00000 . 1.00000 1.00000 1.00000 1.00000 1.00000 . VUY0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000VLY 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.000000.00000 REFRACTNE INDICES GLASS CODE 157.63 ‘cafl vuv’ 1.559288 Nosolves defined in system INFINITE CONJUGATES EFL −521.5384 BFL −94.3531FFL 2582.5092 FNO 0.0000 AT USED CONJUGATES RED −0.2000 FNO −0.6667 OBJDI5 25.0000 TT 1249.8815 IMG DIS 9.9306 OAL 1214.9509 P. SRAXIAL IMAGEHT 25.0018 THI 9.9619 ANG 0.0000 ENTRANCE PUPIL DIA 0.3034E+10 THI0.1000E+11 EXTf PUPIL DIA 155.2520 THI −94.3531The optical design description provided herein demonstrates anadvantageous catadioptric projection system for DUV or VUVphotolithography. While the preferred embodiment has been designed foruse in an 157 nm tool, the basic concept has no wavelength limitations,either shorter or longer, providing a suitable refractive materialexists. Some features of the preferred system herein are summarizedbelow.

Configuration

The preferred optical system is catadioptric and includes two opticalgroups, group G1 and group G2, configured such that group G1 presents areduced, virtual image to group G2. The function of group G2 is to relaythis virtual image to a real image located at the plane of the wafer.Group G1 preferably includes an even number of at least four andpreferably 4 or 6 mirrors in combination with lens elements whoseprimary function is to provide telecentricity at the mask and enablecorrection of axial chromatic aberration. In the preferred embodiment,an image of the aperture stop is located in close proximity to mirrorM2.

Group G2 is preferably entirely dioptric providing most of the systemreduction and a corresponding high numerical aperture (in excess of0.65, 0.70 and even 0.75) at the image. This group G2 also makes thefinal image telecentric in image space. Group G1 functions to correcthigh-order field aberrations, advantageously allowing a substantialrelaxation of the lens elements found in group G2. Both group G1 andgroup G2 make use of aspheric surfaces as set forth in the Table 2. Thesame holds for the second preferred embodiment.

Symmetry

The preferred optical design herein is co-axial, wherein each of theoptical elements is rotationally symmetric about a common centerline.The preferred system advantageously does not utilize fold mirrors,prisms, or beamsplitters to fold the opto-mechanical axis. This enablesa compact configuration and eliminates substantial bulk refractivematerial that may be difficult to procure in a timely manner.

Parallel Scanning

The preferred optical system herein achieves mask and wafer planes thatare parallel, enabling unlimited scanning in a step/scan lithographicconfiguration.

Correction of Chromatic Aberration

Correction of chromatic aberration is achieved preferably using a singleoptical material in the catadioptric configuration described herein.Lateral chromatic aberration is at least substantially self-correctedwithin group G2, using a balance of optical power on either side of aprimary aperture stop located within group G2. Correction of axialchromatic aberration is enabled using a negative lens group E2/E3located at mirror M2 in group G1, providing an axial chromaticaberration contribution that is nearly equal in magnitude and oppositein sign to the chromatic aberration generated by G2. This high level ofaxial chromatic aberration correction relaxes the need for a highspectral purity laser exposure source with linewidths on the order of0.1 to 0.2 pm.

Some additional features of the preferred system herein are set forthbelow. The preferred system is an imaging system for photolithographicapplications using 157 nm, 193 nm or 248 nm or other exposure radiationincluding first and second optical groups, or groups G1 and G2. Thefirst optical group, i.e., group G1, is either a catoptric orcatadioptric group including preferably six mirrors. Group G1 preferablyalso includes one or more lens elements, e.g., to make the chief raytelecentric at a mask plane and to correct axial chromatic aberration.

The second optical group, or Group G2, is a dioptric group of severallens elements for reducing and projecting an image to a wafer plane.Group G2 is preferably a relaxed group such that optical paths ofprojected rays are smoothly redirected at each lens element, e.g., lessthan 45° and preferably less than 30°, and still more preferably lessthan 20°, as shown in FIG. 1. This preferred system is contradistinctform a Dyson-type system which has one reflective component performingreduction of the image. In contrast to the Dyson-type system, thepreferred system has a dioptric second group (group G2) performingreduction, while the catoptric or catadioptric first group (group G1)forms a virtual image for reduction by Group G2 and provides aberrationcompensation for group G2.

The first and second groups, or groups G1 and G2, respectively, of thepreferred imaging system herein enable parallel scanning and asymmetric, coaxial optical design. Stops are located preferably at ornear the second mirror M2 of Group G1 and within Group G2. The firststop may be alternatively moved off of the second mirror to enhanceaberration correction.

Group G2 is preferably independently corrected for lateral color, whilethe refractive components of the first group compensate those of thesecond group far longitudinal color. Advantageously, 15 or fewer totallens elements are preferably included in the system, group G2 preferablyhaving 10 or fewer lens elements. For example, the system of FIG. 1shows ten lens elements E4-E13 in group G2 and three additional lenselements in group G1.

The sixth or final mirror in group G1 may be preferably a convex mirrorand preferably a virtual image is formed behind the sixth mirror. GroupG2 forms a real image at the wafer plane.

When used with 157 nm exposure radiation, all of the refractive elementsof the imaging system. e.g., lens elements E1-E13 of the preferredsystem of FIG. 1, are preferably made from a VUV transparent materialsuch as CaF2. Alternatively, such materials as BaF2, SrF2, MgF2 or LiFmay be used.

While exemplary drawings and specific embodiments of the presentinvention have been described and illustrated, it is to be understoodthat the scope of the present invention is not to be limited to theparticular embodiments discussed. Thus, the embodiments shall beregarded as illustrative rather than restrictive, and it should beunderstood that variations may be made in those embodiments by workersskilled in the arts without departing form the scope of the presentinvention as set forth in the claims that follow, and equivalentsthereof. In addition, the features of different claims set forth belowmay be combined in various ways in further accordance with the presentinvention.

1-11. (canceled)
 12. A system, comprising: a plurality of opticalelements arranged along an optical axis between an object plane and animage plane, the plurality of optical elements including an even numberof mirrors where each mirror defines a corresponding surface that isrotationally symmetric about the optical axis, each surface intersectingthe optical axis at a corresponding intersection point, wherein: theplurality of optical elements define a pupil plane which intersects theoptical axis at a position that is closer to an intersection point of aone of the mirrors than to any of the other optical elements, and theplurality of optical elements are components of a catadioptricmicrolithography projection objective.
 13. The system of claim 12,wherein the plurality of optical elements includes at least fourmirrors.
 14. The system of claim 12, wherein the plurality of opticalelements includes at least six mirrors.
 15. The system of claim 12,wherein the plurality of optical elements comprise at least one lensarranged relative to the other optical elements so that during operationradiation directed from the object plane to the image plane makes adouble pass through the at least one lens.
 16. The system of claim 15,wherein the at least one lens is adjacent to one of the mirrors in theradiation path from the object plane to image plane.
 17. The system ofclaim 15, wherein the at least one lens is adjacent in the radiationpath to the mirror whose corresponding intersection point is closest tothe pupil.
 18. The system of claim 15, wherein the at least one lens isa negative lens.
 19. The system of claim 12, wherein the plurality ofoptical elements includes a plurality of lenses and all of the lensesare formed from the same material.
 20. The system of claim 12, whereinthe plurality of optical elements includes at least one lens formed fromCaF₂.
 21. The system of claim 20, wherein the lens closest to the imageplane is formed from CaF₂.
 22. The system of claim 12, wherein thesystem is configured to image a ring- shaped field at the object planeto the image plane.
 23. The system of claim 12, wherein the system isconfigured to image a rectangular-shaped field at the object plane tothe image plane.
 24. The system of claim 12, wherein the system isconfigured to image an off-axis field at the object plane to the imageplane.
 25. The system of claim 12, wherein the catadioptric projectionobjective has an image-side numerical aperture of 0.65 or more.
 26. Thesystem of claim 12, wherein the catadioptric projection objective has animage-side numerical aperture of 0.7 or more.
 27. The system of claim12, wherein the catadioptric projection objective has an image-sidenumerical aperture of 0.75 or more.
 28. The system of claim 12, whereinthe catadioptric projection objective has an image-side numericalaperture of 0.8 or more.
 29. The system of claim 12, wherein the mirrorscomprise a pair of concave mirrors that are adjacent to one another in apath of the radiation from the object plane to the image plane.
 30. Thesystem of claim 29, wherein during operation the catadioptric projectionobjective forms an intermediate image of an object at the object planebetween the pair of concave mirrors.
 31. The system of claim 12, whereinthe catadioptric projection objective is a dry catadioptric projectionobjective.
 32. The system of claim 12, wherein the optical axis is astraight optical axis.
 33. A system, comprising: a plurality of opticalelements arranged along an optical axis between an object plane and animage plane, the plurality of optical elements including a plurality oflenses and an even number of mirrors where each mirror defines acorresponding surface that is rotationally symmetric about the opticalaxis, each surface intersecting the optical axis at a correspondingintersection point, wherein: the plurality of optical elements define apupil plane which intersects the optical axis at a position that iscloser to an intersection point of a one of the mirrors than to any ofthe other optical elements, at least one of the lenses is positioned ina double pass configuration close to the pupil plane, and the pluralityof optical elements are components of a catadioptric microlithographyprojection objective.
 34. A system, comprising: a plurality of opticalelements arranged along a straight optical axis between an object planeand an image plane, the plurality of optical elements including aplurality of mirrors, wherein: the plurality of optical elements definea pupil plane that is closer one of the mirrors than to any of the otheroptical elements as measured along the optical axis, and the pluralityof optical elements are components of a catadioptric microlithographyprojection objective.
 35. A system, comprising: a plurality of opticalelements arranged along an optical axis between an object plane and animage plane, the plurality of optical elements including an even numberof mirrors, wherein: the plurality of optical elements define a pupilplane that is closer one of the mirrors than to any of the other opticalelements as measured along the optical axis, and the plurality ofoptical elements are components of a catadioptric microlithographyprojection objective.
 36. A system, comprising: a plurality of opticalelements arranged along an optical axis between an object plane and animage plane, the plurality of optical elements including one or morepositive lenses and an even number of mirrors, where each mirror definesa corresponding surface that is rotationally symmetric about the opticalaxis, each surface intersecting the optical axis at a correspondingintersection point, wherein: the plurality of optical elements define apupil plane which intersects the optical axis at a position that iscloser to an intersection point of a one of the mirrors than to any ofthe one or more positive lenses, and the plurality of optical elementsare components of a catadioptric microlithography projection objective.